1. Field of the Invention
This invention relates to a photosensitive member used in electrophotographic apparatus, a process for its production, and an electrophotographic apparatus having this photosensitive member as a light-receiving member. More particularly, this invention relates to an amorphous-silicon (a-Si) type photosensitive member having an amorphous-carbon (a-C) surface protective layer; the photosensitive member having been so improved as to prevent occurrence of faulty images caused by the presence of protrusions standing uncovered to its surface and occurrence of any difficulties or troubles in the step of cleaning the light-receiving member surface in the course of forming electrophotographic images; and also relates to a process for producing such a photosensitive member, and an electrophotographic apparatus having such a photosensitive member as a light-receiving member and not causative of any faulty images and any difficulties or troubles in the cleaning step.
2. Related Background Art
In electrophotographic apparatus such as copying machines, facsimile machines and printers, first the periphery of a photosensitive member comprising a conductive cylindrical substrate provided on its surface with a photoconductive layer is uniformly electrostatically charged by the use of charging means such as corona charging, roller charging, fur brush charging or magnetic-brush charging. Next, light reflecting from an image to be copied, of an original document, or laser light or LED light corresponding to modulated signals of that image is used to expose the photosensitive member surface to form an electrostatic latent image on the periphery of the photosensitive member. Then, a toner is made to adhere to the photosensitive member surface to form a toner image from the electrostatic latent image, and the toner image is transferred to a copying paper or the like, thus a copy is taken (image formation).
After the copy has been taken in this way, the toner remains partly on the periphery of the photosensitive member, and hence such residual toner must be removed before the next copying step is carried on. Such residual toner is commonly removed by means of a cleaning unit making use of a cleaning blade, a fur brush or a magnet brush.
In recent years, in consideration of environment, electrophotographic apparatus are also proposed in which the above cleaning unit making use of a mechanical removal method is omitted for the purpose of reducing waste toner or eliminating waste toner, and some have already been on the market. The residual-toner removal method used in this electrophotographic apparatus includes, e.g., a method in which a direct-charging assembly such as a brush charging assembly as disclosed in Japanese Patent Application Laid-Open No. 6-118741 is used to carry out both a cleaning step and a charging step, and a method in which a developing assembly as disclosed in Japanese Patent Application Laid-Open No. 10-307455 (corresponding to U.S. Pat. No. 6,128,456) is used to carry out both a cleaning step of collecting the residual toner and a developing step of making the toner adhere. Either of the above cleaning methods has a step in which the toner and the photosensitive member surface are brought into rubbing friction to remove the toner.
Meanwhile, in recent years, in order to achieve higher image quality of printed images, it is put forward to use toners having a smaller average particle diameter than ever or to use toner having a lower melting point so as to be adaptable to energy saving. At the same time, with advancement of surrounding electric circuit devices, the copying speed of electrophotographic apparatus, i.e., the number of revolutions of photosensitive members is being made higher and higher. Under such circumstances, with an increase in the copying speed and frequency of electrophotographic apparatus, a phenomenon has come to occur in which the residual toner causes its melt adhesion to the photosensitive member surface. In particular, in recent years, with advancement of digitization of electrophotographic apparatus, the demand on image quality is more and more raised in level to have reached a situation that even image defects at a level tolerable in conventional analog-type apparatus must be regarded as questionable. Accordingly, it is demanded to remove factors causative of such image defects and, in respect of the occurrence of melt adhesion caused by the residual toner, too, to take any effective countermeasures for eliminating or preventing it.
The cause of the occurrence of melt adhesion or filming has not been elucidated in detail, but its occurrence is roughly estimated to be due to the following factors. For example, in the cleaning step making use of a cleaning blade or the like, the frictional force acting between the photosensitive member and the part rubbing against it (rubbing part) may cause a phenomenon of chattering in the state of contact. With this phenomenon, the effect of compression against the photosensitive member surface may become higher, so that the residual toner may strongly be pressed against the photosensitive member to cause the melt adhesion or filming. In addition, with an increase in process speed for the image formation of electrophotographic apparatus, the relative speed between the rubbing part and the photosensitive member increases more and more, and hence this also makes it tend to bring about the situation for the cause of occurrence.
As countermeasures for keeping the melt adhesion or filming from occurring, which is caused by the frictional force acting between the photosensitive member and the rubbing part, it is proposed, as disclosed in Japanese Patent Application Laid-Open No. 11-133640 (corresponding to U.S. Pat. No. 6,001,521) and Japanese Patent Application Laid-Open No. 11-133641 (corresponding to U.S. Pat. No. 6,001,521), that an amorphous carbon layer containing hydrogen (hereinafter “a-C:H film”) is used as a surface protective layer of a photosensitive member, and such a layer is shown to be effective. This a-C:H film, as it is also called diamond-like carbon (DLC), has a very high hardness. Hence, it can prevent scratches and wear and in addition thereto has a peculiar solid lubricity. From these two characteristics, it is considered to be an optimum material for preventing the melt adhesion or filming.
However, this a-C:H film and an amorphous silicon (hereinafter “a-Si”) film used in a photoconductive layer may differ in optimum production conditions. More specifically, in the case of a-Si photosensitive members, it is common to set substrate temperature to 200° C. to 450° C. in order to attain practical characteristics. On the other hand, in the case of the a-C:H film, it is better for the substrate temperature to be set low to obtain a good film, and hence, the film is often formed setting the substrate temperature at room temperature to about 150° C. Accordingly, when a surface layer comprised of a-C:H is deposited on a photosensitive member having a photoconductive layer formed basically of a-Si, it has been necessary to lower to room temperature to about 150° C. the substrate temperature set to 200° C. to 450° C., and thereafter form the a-C:H surface layer. In many deposition chambers, a heater for heating substrates is built in to control the temperature of substrates, but, in many cases, any member for cooling is not provided. Accordingly, it has been inevitable to rely on natural heat dissipation in order to lower to room temperature to about 150° C. the substrate temperature having been kept at 200° C. to 450° C., so that it has taken a very long time especially in vacuum environment. Hence, there has been a problem that photosensitive members are producible only in a small number per day per one deposition chamber, resulting in a cost increase for the manufacture of photosensitive members.
As another problem, when the photosensitive members thus produced taking a long time are inspected for shipment after their completion, defectives may occur which make products unacceptable, because of unexpected poor image formation or poor potential. Such occurrence of defectives has also been a factor for the cost increase.
Apart from the foregoing, in the case of a-Si photosensitive members, as a problem on their production processes, it is also known, as disclosed in Japanese Patent Application Laid-Open No. 62-189477, that protrusions often occur at the surfaces of deposited films. Many proposals are made on how to keep such protrusions from occurring, but it is considered very difficult in respect of techniques and also in respect of cost to make the protrusions not occur at all which arise from minute foreign matter having accidentally adhered to the surface.
At the part of such protrusions, the melt adhesion of a developer (toner particles) tends to occur. Even in an attempt to use the a-C:H film in the surface protective layer to keep the melt adhesion from occurring at normal areas except the protrusions, it has not been made able to perfectly prevent so far as the occurrence of melt adhesion at the part of protrusions.
In addition, the photosensitive member is, when used inside the electrophotographic apparatus, rubbed with any members coming into contact therewith and becomes worn, in the course of charging, development, transfer and cleaning. In that course, compared with the part of normal areas, the part of protrusions may selectively greatly wear because of its peculiarity in shape. Moreover, what has not been image defects at the initial stage may come to image defects because of a lowering of charge retentivity as a result of the wearing at the part of vertexes of the protrusions. Also, the part having worn at the protrusion vertexes comes not having any surface protective layer formed of a-C:H film (hereinafter often simply “a-C surface layer”) to cause melt adhesion at that part as the starting point. Thus, such wearing has a possibility of coming to a factor which deteriorates image characteristics.
In this connection, in a system where the chief cause of wear is the rubbing friction acting in the cleaning step, the wear at the part of normal areas is at a level of about 1 nm per 10,000 sheets when an amorphous silicon carbide (a-SiC) surface layer is used. Also, in a system where the chief cause of wear is a contact charging step involving a high rubbing frictional force, the wear at the part of normal areas is at a level of about 10 nm per 10,000 sheets in the case of the a-SiC surface layer, whereas, it is approximately at a level of about 1 nm per 10,000 sheets in the case of the a-C surface layer.
In addition, in a system where a cleaning blade is commonly used, the blade may be damaged or broken off because of the protrusions to cause what is called the developer (toner) escape, so that there is also a possibility of causing faulty cleaning.